The present invention to monitors the integrity of fissile materials, particularly to a fissile material detector, and more particularly to a fissile material detector having a sample cavity capable of monitoring the content of fissile materials in large items, uses neutron sources fabricated in spatially extended shapes mounted in endcaps of the sample cavity, a thermal neutron filter insert, and a neutron reflector insert, and a neutron multiplicity coincidence counter.
Various types of detectors for fissile materials are known, as exemplified by U.S. Pat. No. 4,201,912, issued May 6, 1980; U.S. Pat. No. 4,510,117 issued Apr. 9, 1985; and U.S. Pat. No. 4,617,466, issued Oct. 14, 1986. Gamma spectrometer detectors, which determine the amount and isotopic composition of fissile materials from the intensity and shape of the measured gamma spectrum, can be considered as an analog. The use of such a prior known detector is limited to small items of homogeneous composition (weight of about 100 grams), since the characteristic gamma radiation of fissile materials is not of very high energy (up to 200 keV) and the penetration depth of such gamma photons is no more than a few millimeters.
Fast neutrons have significant penetrating ability (20-30 cm) for irradiating items made from fissile materials. So containers of about 200 liters volume may be scanned by the system such as the active-well neutron coincidence counters, exemplified by a Model JCC-51, made by Canberra Industries, Inc., Meriden, Conn, in which the sample cavity is monitored by 3He-filled neutron detectors. Also see xe2x80x9cActive Nondestructive Assay of Nuclear Materials, Principles and Applicationsxe2x80x9d January 1981. The characteristic neutron emission upon spontaneous fission of such materials as uranium-235 and plutonium-239 is extremely low, and so the above-referenced neutron detectors of fissile materials are mainly used to detect plutonium-240 and small uranium-235 samples.
Thus, a need has existed for a fissile material detector which can be effectively utilized to detect uranium-235 and plutonium-239, and wherein the volume of the sample cavity is sufficiently large to enable nondestructive assay of large items of arbitrary configuration.
The present invention provides a solution to the above-referenced need by providing a fissile material detector having operating principles very close to the above-referenced active-well coincidence counter.
In the detector of the present invention, the assay sample is placed in a cylindrical cavity of 35 liters volume, for example, surrounded by a polyethylene neutron moderator, with 3He-filled neutron counters positioned within the moderator. A thermal neutron filter insert of boron carbide extends around the cavity and is key to increasing the energy of the neutron flux in the cavity, thus improving uniformity of sample irradiation. The sample is irradiated by distributed Amxe2x80x94Li neutron sources from both endcaps of the cavity, inducing fission uniformly in uranium-235 or plutonium-239.
It is an object of the present invention to provide an improved fissile material detector.
A further object of the invention is to provide a fissile material detector having a sample cavity volume large enough to make non-destructive assay possible for large items of arbitrary configuration.
A further object of the invention is to provide a fissile material detector for integrity monitoring of fissile materials, and for nondestructive assay to confirm the presence of a stable content of fissile materials in items.
Another object of the invention is to provide a fissile material detector having a sample cavity volume increased by about an order of magnitude.
Another object of the invention is to provide a fissile material detector which utilizes Amxe2x80x94Li neutron sources fabricated in spatially extended shapes, such as flat rings, and mounted on the endcaps of the sample cavity.
Another object of the invention is to provide a fissile material detector which utilizes Amxe2x80x94Li neutron sources composed of a large americium layer (i.e. xcx9c75 cm) pressed against a large lithium layer.
Another object of the invention is to provide a fissile material detector which utilizes a thermal neutron insert with reflector properties, such as boron carbide, located around the sample cavity to provide higher penetration neutrons for a more uniform neutron interrogation.
Another object of the invention is to provide a fissile material detector having an electronics module which includes a neutron multiplicity coincidence counter.
Another object of the invention is to provide a fissile material detector wherein the neutron emission enables use with materials such as uranium-235 and plutonium-239 in large sizes.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically the invention involves a fissile material detector capable of detecting uranium-235 and plutonium-239. Thus, the invention relates to monitoring the integrity of various fissile materials and may be used for nondestructive assay to confirm the presence of a stable content of fissile materials in items or objects. The detector of this invention utilizes a sample cavity which is about an order of magnitude larger than any known active well coincidence counter. The detector utilizes Amxe2x80x94Li neutron sources fabricated in spatially extended shapes, such as flat rings, mounted on the endcaps of the sample cavity. The detector utilizes a thermal neutron filter about the sample cavity, which may be made of boron carbide and which functions to block the thermal neutron flux from the sample cavity, and as a reflector for fast neutrons, increasing their flux and uniformity inside the cavity. The electronics module of the detector includes a neutron multiplicity coincidence counter, which consists of an adder, an adjustable delay generator, a binary counter, a register, and a controller, and thus it is possible to compile a multiparameter assay certificate for the item under test.